Mod_pagespeed waiting for completion

So I've done the Get Started with Mod_pagespeed tutorial to get it up and running on my server. Its been running fine, but recently, I did a server update and I've started to see these errors in my error log:

Your droplet will likely see all kinds of processes being killed and things you don't expect when you exhaust all your available memory. You can use the command free -m to view the available memory on your system and you may want to consider upgrading to a larger droplet or trying a swap file so out of memory errors don't persist.

Yea that makes total sense, I just had 100 people on the server at once when those errors started popping up + I'm running a Wordfence security plugin and jet pack and both of those eat up quite a lot of resources I've been noticing. When I resize the droplet tonight, I have to power it off - how long does it usually take to resize?